Solid electrolytic capacitors of interest to this invention include a capacitor as described in, for example, Japanese Patent No. 4439848 (Patent Document 1). Patent Document 1 discloses, as an embodiment, a solid electrolytic capacitor structured as shown in FIG. 4.
Referring to FIG. 4, the solid electrolytic capacitor 1 includes a laminated body 3 that has a plurality of capacitor elements 2 laminated. Each capacitor element 2 includes a valve-acting metal base 6 that has an aluminum layer 4 as a core part and a porous layer 5 as a rough surface part, which is formed on the surface of the aluminum layer 4. On the surface of the porous layer 5, a dielectric coating film (not shown) is formed, a solid electrolyte layer 7 is formed on the dielectric coating film, and a collector layer 8 is formed on the solid electrolyte layer 7.
The plurality of capacitor elements 2 bonded to each other with a conductive adhesive 9 interposed therebetween constitute the laminated body 3. The conductive adhesive 9 mechanically fixes the adjacent capacitor elements 2 to each other, and electrically connects the respective collector layers 8 of the adjacent capacitor elements 2 to each other. For example, a negative electrode terminal member 10 is inserted between specific capacitor elements 2, and electrically connected to the collector layers 8.
The laminated body 3 is covered by an exterior 11 of an electrically insulating resin. In this case, one end surface 12 of the valve-acting metal base 6 is adapted to be exposed on a first end surface 13 of the exterior 11, and one end surface 15 of the negative electrode terminal member 10 is adapted to be exposed on a second end surface 14 opposed to the first end surface 13 of the exterior 11.
A positive-electrode external electrode 16 is provided on the first end surface 13 of the exterior 11, whereas a negative-electrode external electrode 17 is provided on the second end surface 14. The positive-electrode external electrode 16 is electrically connected to the aluminum layer 4 of the valve-acting metal base 6, whereas the negative-electrode external electrode 17 is electrically connected to the collector layer 8 with the negative-electrode terminal member 10 interposed therebetween.
On the end surface 12 of the valve-acting metal base 6, a zinc layer 18 is formed which serves as a part of the positive-electrode external electrode 16. The zinc layer 18 is formed by zinc substitution in a way that the aluminum layer 4 and dielectric coating film exposed from the exterior 11 are immersed in a sodium hydroxide solution containing zinc, or a hydrofluoric acid or ammonium fluoride solution containing zinc. On the zinc layer 18, a first nickel layer 19 is formed which serves as a part of the positive-electrode external electrode 16. The first nickel layer 19 is formed by immersion in a solution containing a nickel salt and a boron hydroxide compound.
The positive-electrode external electrode 16 further has a second nickel layer 20 on the first nickel layer 19, a copper layer 21 thereon, a third nickel layer 22 thereon, and a tin layer 23 thereon. The second nickel layer 20 is formed by an electroless plating method, and the copper layer 21, the third nickel layer 22, and the tin layer 23 are formed by an electrolytic plating method.
The negative-electrode external electrode 17 includes the second nickel layer 20, the copper layer 21 thereon, the third nickel layer 22 thereon, and the tin layer 23 thereon, which are also included in the positive-electrode external electrode 16. As is the case in the positive-electrode external electrode 16, the second nickel layer 20 in the negative-electrode external electrode 17 is formed by an electroless plating method, whereas the copper layer 21, the third nickel layer 22, and the tin layer 23 are formed by an electrolytic plating method.
Patent Document 1: Japanese Patent No. 4439848